Fibrous Aerogels with Tunable Superwettability for High-Performance Solar-Driven Interfacial Evaporation

Highlights Hybrid fibrous aerogels with tunable wettability from the same molecular unit of vinyltrimethoxysilane are successfully developed. Superhydrophobic and superhydrophilic hybrid aerogels are integrated into a double-layered evaporator, showing robust interfacial networks to withstand repeated and tremendous compression for 1000th cycle. The evaporator delivers high water evaporation rates of 1.91 kg m−2 h−1 under laboratory conditions and 4.20 kg m−2 h−1 under outdoor experiments with the aid of wind (1 sun), enabling efficient salt rejection under continuous operation. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01034-4.

Si NMR spectra and water contact angle photos for monitoring polycondensation process of VTMS. Water contact angle results show hydrophobicity of VNFs aerogel was originated at the initial process of materials formation, demonstrating molecular orientation of siloxane was occurred when they contact with BC nanofibers Nano-Micro Letters S4 / S18

Fig. S5 a-b
Solid-state 29 Si NMR spectra and water contact angle photos for monitoring the polycondensation process of PVTMS. During the initial process, the structure of the preliminarily-developed aerogel was easily collapsed after water invasion. As polycondensation proceeded, the structure of PNFs became robust and showed negligible deformation after water droplet absorption. The results highlight the capability of polysiloxane that could strengthen the aerogel network  Controlled experiments for the explanation of superwettability of the aerogels. a Schematic illustration of composite films fabricated by dripping the mixture of BC nanofibers and VTMS sol (or PVTMS sol) onto a glass slide, then annealing process at 80 o C for 12 h was applied. Finally, VNFs and PNFs films without porous structure were obtained. b Water contact angle of VNFs and PNFs films is 92 and 53, respectively. The results indicate that surface roughness is important to afford aerogels with superwettable property. c Water contact angle of pure PVSQ and PVPSQ film is 104 and 63, respectively, revealing different molecular orientation of polysiloxane (PVSQ and PVPSQ) for distinct surface wettability. d Microscopic structure of VNFs film and PNFs film at different magnifications observed by SEM showing the isotropic fibrous structure that constituted by close-packed nanofibers From the results, stronger and more stiff mechanical properties were observed in axial direction, showing anisotropic mechanical character of the aerogels. Along the pore alignment, the maximum stress of VNFs and PNFs at the strain of 80% was 20.0 and 41.8 kPa, respectively, indicating that the hybrid aerogels could withstand more than 17,000 times and 27,000 times their own weights.      The heights of top evaporative layer (PNFs-PPy, h1) and bottom insulation part (VNFs, h2) were optimized by evaluating evaporation rates and evaporation efficiencies. a Water evaporation rates and efficiencies of the evaporators with various architecture. The height of PNFs-PPy (h1) is fixed to be 1.0 cm, and h2 is variable. The results revealed that the optimized performance was achieved when h2 = 2.0 cm. b Water evaporation rates and efficiencies of the evaporators with various architecture. Height of VNFs (h2) was determined to be 2.0 cm, and h1 is variable. Based on the data in Fig. S16b, optimized h1 and h2 were determined to be 1.0 and 2.0 cm for evaporator construction. The evaporation efficiencies of above samples are calculated according to Eq. 1 (main text) and the evaporation rate in dark situation was subtracted (0.43 kg m -2 h -1 for all double-layered evaporators).   Water evaporation rates during 6 h of solar desalination at outdoor windless environment. The experiment was conducted in a closed system from 10:00-15:00 under natural sunlight. It is noted that the evaporation rate maximum could reach 1.03 kg m -2 h -1 , which is much lower than that under windy environment (4.02 kg m -2 h -1 ). On one hand, the air movement allowed the water to evaporate more freely in the open system. On the other hand, open system allowed complete sunlight incidence onto the evaporator surface without energy loss. Both reasons account for higher evaporation rate in an open system under a windy environment. S12 / S18

Note S1 Porosity Calculations
The porosity was calculated according to Eq. S1: where ρ is the bulk density of aerogel, which can be calculated by the weight and volume of aerogel. ρskeleton is the density of solid skeleton of aerogel, which can be calculated from Eq. S2: Wsi is the mass fraction of polysiloxane in the aerogel, which was calculated from Eq. S3: where mBC (0.0236 g) and m are the weight of the BC nanofibers (pure BC aerogel) and the entire aerogels (VNFs or PNFs), respectively. ρBC = 1.50 g cm -3 , ρVNFsSi = 1.18 g cm -3 , and ρPNFsSi = 1.50 g cm -3 , which were calculated referring to the literature [S1, S6, S7]. As a result, the porosity of VNFs and PNFs was calculated to be 99.6% and 99.5%, respectively.

Note S2 Simulation Details and Methodology
Classic molecular dynamics simulations were carried out to investigate the molecular orientation of polysiloxane on the surface of BC nanofibers at the atomic level. Two kinds of macromolecules named PVPSQ and PVSQ were studied in this project, which contained saturated carbon chains and vinyl groups, respectively. The molecular structures of PVPSQ, PVSQ, and cellulose are shown in the Figure 3 (main text). Two simulations were performed, in which thirty-five PVSQ molecules (or thirty-five PVPSQ molecules) and thirty-five cellulose molecules were packed in the cubic simulation box with the software of PACKMOL, respectively [S8]. The detailed information about the simulation boxes was listed in Table S1 and referred to the previous study [S9]. For each simulation, energy minimization was firstly employed to relax the simulation box. Next, an isothermal-isobaric (NPT) ensemble with a 1.0 fs time step was taken to optimize the simulation box, where the temperature and pressure were set to be 298.15 K and 1.0 atm, respectively. The Nose-Hoover thermostat and Parrinello-Rahman barostat were used to stabilize the temperature and pressure of the simulation box. Following the NPT simulation, a canonical (NVT) ensemble with a 30.0 ns time step was performed, in which the prior 20.0 ns was used to optimize the simulation box and the later 10.0 ns was used to collect the trajectory coordinates of molecules. All simulations were carried out using the GROMACS 2019.5 package [S10].

Note S3 Water Transportation Rate Calculation
According to Movie S2, the transport distance of 2.0 cm could be reached (diameter = 1.0 cm) within 2.0 s. Therefore, the volume of water transportation through the water channel could be calculated with Eq. S4: The rate of water transportation in PNFs channel was 2827 cm 3 h -1 .
On the other hand, the evaporation rate was 1.91 kg m -2 h -1 , and the diameter of evaporation surface was 5.50 cm, the evaporation rate of the individual evaporator reached 859 cm 3 h -1 . Therefore, water supply of the system is sufficient for evaporation, which is important for highly efficient solar evaporator.

Note S4 Energy Loss Analysis
Energy loss of evaporator was analyzed referring to the literature [S11, S12].
According to above data, the radiation loss of double-layered evaporator was calculated to ~3.0%, and ~13.7% for single-layered PNFs-PPy.
(2) Conduction loss was calculated by Eq. S6: Q is the heat energy, C is the specific heat capacity of pure water (= 4.2 kJ °C −1 kg −1 ), m is the weight of bulk water, ΔT is the temperature difference between the bulk water before and after experiment. In our experiment, the weight of bulk water was 60.0 g, the temperature difference of double-layered evaporator reached 0.8 o C, while 6.0 o C for single-layered PNFs-PPy. According to above data, the conduction heat loss of the double-layered evaporator was calculated to be ~2.3%, and ~17.6% for single-layered PNFs-PPy.
(3) Convection loss was calculated by Eq. S7: h is the convection heat transfer coefficient (~5 W m −2 K −1 ), Asurface is the surface aera of evaporator, ΔT is the temperature difference between evaporator surface and ambient environment. According to above data, the convection loss of double-layered evaporator was calculated to be ~2.3%, and ~10% for the single-layered PNFs-PPy.
(4) Reflection loss: The absorption of evaporator according to UV-vis-NIR diffuse reflectance spectroscopy (Figure 4c in main text) is 95%; thus, the refection loss of double-layered evaporator and single-layered PNFs-PPy is ∼5%.

Note S5 Water Evaporation Enthalpy Calculation
The water vaporization enthalpy of water-filled PNFs-PPy aerogel was determined by the equivalent evaporation enthalpy measurement. The experimental details were referred to the literature and calculated using Eq. S8 [S13]: where Δ is the evaporation enthalpy, and 0 is the mass change of bulk water; Δ and are the equivalent evaporation enthalpy and mass change of waterfilled PNFs-PPy aerogel, respectively. According to the above equation, the water evaporation enthalpy of water in water-filled PNFs aerogel was calculated to be 2068 J g -1 .

Note S6 Optimized Geometry
The height of VNFs and PNFs-PPy was determined by gravity, heat location performance, and salt rejection performance referring to the literature [S11, S14].
The single-layered PNFs-PPy evaporator was immersed in water throughout the evaporation process, resulting in heat loss to bulk water. As a result, VNFs was introduced to support PNFs-PPy. The force required to support PNFs-PPy was derived from the water excluded by VNFs, and was calculated by Eq. S9: Fb represents the buoyancy provided by VNFs, Gex is gravity of the water (Vex: volume of water) excluded by the double-layered evaporator, and ρw corresponds to density of water. The critical condition of the suitable design is that Fb should be equal to Gex. Therefore, the following Eqs. S10-S12 are derived: In this study, ρ1 = 1.00 g cm -3 , ρ2 = 6.00 mg cm -3 . hex can be simplified using Eq. S13: In our system, r1 = 0.50 cm, r2 = 2.75 cm, h1 = 1.00 cm. hex was calculated to be 1.04 cm. Therefore, as long as h2  1.04 cm, solar absorber is able to isolate the bulk water from PNFs-PPy layer.
From the point of heat location, the thickness of insulator is important in reducing heat conduction from top to bulk water and determining the height of water path. Heat transferred by the conduction process could be expressed by the Eqs. S14 and S15: Q is the heat transferred through the system, λ is the thermal conductivity of VNFs, Δt is the temperature difference between top and bottom of evaporator, A is the area of the surface, and Δx is the thickness of VNFs.
In this system, we define that the heat conduction loss through the insulator could be less than 5%, and the temperature difference between top and bottom is 10 o C, that is ≤ % , = ℃ Therefore, Δx should be larger than 0.60 cm, which is also the length of water path.

≥ .
For salt rejection performance, the length of water path determines the diffusion performance of salt ions (Jdiff, kg m -2 s -1 ) according to Eq. S16 (Fick's law of diffusion) [S14] : D is 1.6  10 -9 m 2 s -1 (NaCl diffusion coefficient in water); Cs and Cb are the ions concentrations in PNFs-PPy and bulk solution (salinity = 25 wt%, saturated brine), respectively; ε is the porosity of the evaporator (= 99.48%); τ is the tortuosity of water path (= 1), is the length of the water path. With a salt excretion rate (Jexcr, kg m -2 h -1 ) on the evaporator under 1 sun illumination (1 kW m -2 ), the solar vapor generation energy conversion efficiency could be calculated by Eq. S17: = − (S14) = (S15) m is the mass flux during evaporation, hlv is the liquid-vapor phase change enthalpy, Jexcr could be calculated by Eq. S18:

= ( ) • (S18)
In order to avoid salt accumulation, Jdiff should be larger than Jexcr, and the final length of water path could be calculated with Eq. S19: As a result, the length of water channel (L) was calculated to be less than 2.35 cm.
Combining the optimized length by calculation of gravity, heat conduction, and salt rejection, the length of water path (h2) should be in the range of 1.04−2.35 cm.